The role of Britain's intelligence service in the rendition and torture of an opponent of former Libyan dictator Muammar Gaddafi is set to be exposed in a case at London's High Court.

The High Court will hear a challenge to the Crown Prosecution service's (CPS) 2014 decision not to prosecute an ex-MI6 director for his role in sending a family to Gaddafi's torture chambers.

The case, which is likely to send shockwaves through the British intelligence community, will hear the application for a judicial review of the CPS's decision not to charge MI6's former counterterrorism director, Sir Mark Allen, over the abduction of Abdel Hakim Belhaj and his pregnant wife, who were transferred to Libya in a joint CIA-MI6 operation in 2004.

The case of Belhaj came to light after the fall of Tripoli in 2011, when faxes from Allen describing the rendition flights were discovered by Human Rights Watch in the ransacked office of Moussa Koussa, Libya's head of intelligence.

In a letter to Koussa, Allen said: "I congratulate you on the safe arrival of [Belhaj]. This was the least we could do for you and for Libya to demonstrate the remarkable relationship we have built over recent years."

He also said: "Amusingly, we got a request from the Americans to channel requests for information from [Belhaj] through the Americans. I have no intention of doing any such thing."

The High Court's decision could mean Belhaj's legal team can apply for confidential documents about MI6's part in the rendition of Belhaj to be shown to the court, a move that could embarrass the CPS.

Mark Allen, former head of MI6's counter-terrorism unit (screengrab)

Dropped due to 'insufficient evidence'

The review comes after the CPS dropped a case against the former MI6 director due to "insufficient evidence" after the London Metropolitan Police submitted a 28,000-page dossier, which suggested there were grounds for Allen to be charged.

Disheartened by the decision, Belhaj told Middle East Eye that he was more determined to pursue the case "to the end" after it was dropped by the CPS.

"I am not sure I will win, but I'm going to stick with it until the end," he said.